Re: [PATCH v6 10/10] Documentation: Add documentation for VDUSE

[Jason Wang <jasowang@redhat.com>]

在 2021/4/15 下午10:38, Stefan Hajnoczi 写道:
> On Thu, Apr 15, 2021 at 04:36:35PM +0800, Jason Wang wrote:
>> 在 2021/4/15 下午3:19, Stefan Hajnoczi 写道:
>>> On Thu, Apr 15, 2021 at 01:38:37PM +0800, Yongji Xie wrote:
>>>> On Wed, Apr 14, 2021 at 10:15 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>>>>> On Wed, Mar 31, 2021 at 04:05:19PM +0800, Xie Yongji wrote:
>>>>>> VDUSE (vDPA Device in Userspace) is a framework to support
>>>>>> implementing software-emulated vDPA devices in userspace. This
>>>>>> document is intended to clarify the VDUSE design and usage.
>>>>>>
>>>>>> Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
>>>>>> ---
>>>>>>    Documentation/userspace-api/index.rst |   1 +
>>>>>>    Documentation/userspace-api/vduse.rst | 212 ++++++++++++++++++++++++++++++++++
>>>>>>    2 files changed, 213 insertions(+)
>>>>>>    create mode 100644 Documentation/userspace-api/vduse.rst
>>>>> Just looking over the documentation briefly (I haven't studied the code
>>>>> yet)...
>>>>>
>>>> Thank you!
>>>>
>>>>>> +How VDUSE works
>>>>>> +------------
>>>>>> +Each userspace vDPA device is created by the VDUSE_CREATE_DEV ioctl on
>>>>>> +the character device (/dev/vduse/control). Then a device file with the
>>>>>> +specified name (/dev/vduse/$NAME) will appear, which can be used to
>>>>>> +implement the userspace vDPA device's control path and data path.
>>>>> These steps are taken after sending the VDPA_CMD_DEV_NEW netlink
>>>>> message? (Please consider reordering the documentation to make it clear
>>>>> what the sequence of steps are.)
>>>>>
>>>> No, VDUSE devices should be created before sending the
>>>> VDPA_CMD_DEV_NEW netlink messages which might produce I/Os to VDUSE.
>>> I see. Please include an overview of the steps before going into detail.
>>> Something like:
>>>
>>>     VDUSE devices are started as follows:
>>>
>>>     1. Create a new VDUSE instance with ioctl(VDUSE_CREATE_DEV) on
>>>        /dev/vduse/control.
>>>
>>>     2. Begin processing VDUSE messages from /dev/vduse/$NAME. The first
>>>        messages will arrive while attaching the VDUSE instance to vDPA.
>>>
>>>     3. Send the VDPA_CMD_DEV_NEW netlink message to attach the VDUSE
>>>        instance to vDPA.
>>>
>>>     VDUSE devices are stopped as follows:
>>>
>>>     ...
>>>
>>>>>> +     static int netlink_add_vduse(const char *name, int device_id)
>>>>>> +     {
>>>>>> +             struct nl_sock *nlsock;
>>>>>> +             struct nl_msg *msg;
>>>>>> +             int famid;
>>>>>> +
>>>>>> +             nlsock = nl_socket_alloc();
>>>>>> +             if (!nlsock)
>>>>>> +                     return -ENOMEM;
>>>>>> +
>>>>>> +             if (genl_connect(nlsock))
>>>>>> +                     goto free_sock;
>>>>>> +
>>>>>> +             famid = genl_ctrl_resolve(nlsock, VDPA_GENL_NAME);
>>>>>> +             if (famid < 0)
>>>>>> +                     goto close_sock;
>>>>>> +
>>>>>> +             msg = nlmsg_alloc();
>>>>>> +             if (!msg)
>>>>>> +                     goto close_sock;
>>>>>> +
>>>>>> +             if (!genlmsg_put(msg, NL_AUTO_PORT, NL_AUTO_SEQ, famid, 0, 0,
>>>>>> +                 VDPA_CMD_DEV_NEW, 0))
>>>>>> +                     goto nla_put_failure;
>>>>>> +
>>>>>> +             NLA_PUT_STRING(msg, VDPA_ATTR_DEV_NAME, name);
>>>>>> +             NLA_PUT_STRING(msg, VDPA_ATTR_MGMTDEV_DEV_NAME, "vduse");
>>>>>> +             NLA_PUT_U32(msg, VDPA_ATTR_DEV_ID, device_id);
>>>>> What are the permission/capability requirements for VDUSE?
>>>>>
>>>> Now I think we need privileged permission (root user). Because
>>>> userspace daemon is able to access avail vring, used vring, descriptor
>>>> table in kernel driver directly.
>>> Please state this explicitly at the start of the document. Existing
>>> interfaces like FUSE are designed to avoid trusting userspace.
>>
>> There're some subtle difference here. VDUSE present a device to kernel which
>> means IOMMU is probably the only thing to prevent a malicous device.
>>
>>
>>> Therefore
>>> people might think the same is the case here. It's critical that people
>>> are aware of this before deploying VDUSE with virtio-vdpa.
>>>
>>> We should probably pause here and think about whether it's possible to
>>> avoid trusting userspace. Even if it takes some effort and costs some
>>> performance it would probably be worthwhile.
>>
>> Since the bounce buffer is used the only attack surface is the coherent
>> area, if we want to enforce stronger isolation we need to use shadow
>> virtqueue (which is proposed in earlier version by me) in this case. But I'm
>> not sure it's worth to do that.
> The security situation needs to be clear before merging this feature.


+1


>
> I think the IOMMU and vring can be made secure. What is more concerning
> is the kernel code that runs on top: VIRTIO device drivers, network
> stack, file systems, etc. They trust devices to an extent.
>
> Since virtio-vdpa is a big reason for doing VDUSE in the first place I
> don't think it makes sense to disable virtio-vdpa with VDUSE. A solution
> is needed.


Yes, so the case of VDUSE is something similar to the case of e.g SEV.

Both cases won't trust device and use some kind of software IOTLB.

That means we need to protect at both IOTLB and virtio drivers.

Let me post patches for virtio first.


>
> I'm going to be offline for a week and don't want to be a bottleneck.
> I'll catch up when I'm back.


Thanks a lot for comments and I think we had sufficent time to make 
VDUSE safe before merging.


>
> Stefan